Start in small chunks. I’m going to assume you know nothing. #1: get a environment to work in. This can be local or hosted. #2: Google how to upload files in html (I would start here). You will need a server-side solution to deal with what people upload (I suggest PHP). You will also really need to look into security. Allowing people to upload stuff can be scary. Really, be careful. Get my drift? This is going to be a lot of work. You can’t click a button and be done. Baby steps. Solve one problem and then work on the next.